A) Field of the Invention
This invention relates to a seal member adapted to effect a seal between a rotatable shaft and a housing through which that shaft extends.
B) Description of the Prior Art
It is frequently necessary to effect a fluid-tight seal between a rotatable shaft and a housing containing the shaft, the housing has a bore through which the shaft extends. There are innumerable designs of seal members intended for this purpose and a machine designer has to select the most appropriate type, having regard to the operating conditions.
In the case of shafts which rotate at relatively high speeds and in the presence of pressurised hot gasses, such as are encountered in the case of gas turbines and jet engines, various designs of brush seals have been developed, on account of the unsatisfactory performance of more simple seals having a synthetic elastomeric annular sealing member which bears on the shaft. In such a brush seal, a plurality of fine bristles (such as of bronze or stainless steel wire) are held in a carrier mounted on the housing, the tips of the bristles wiping against the shaft so as to effect a seal thereagainst.
Experience shows that though a brush seal as described above initially may perform satisfactorily, the efficiency of the seal tails off after an extended period of use. This has been found to be due, at least in part, to the bristles losing apparent flexibility on account of debris accumulating in the spaces between individual bristles. That problem may be compounded by the debris oxidizing and then occupying an even greater volume. In an attempt to reduce this problem, it has been suggested that each bristle should be of an elliptical cross-section, rather than of a strictly circular cross-section, in an attempt to increase the packing density of the bristles and so to reduce the volume of the interstices between the bristles. Though a seal with elliptical bristles may give a slightly improved performance, nevertheless it is still prone to a loss of efficiency after an extended period of time.
A further problem with brush seals having bristles of a circular cross-section is that the bristles tend to flex in the axial direction, away from the high pressure side of the seal towards the low pressure side. Bending of the bristles in this way reduces the effective radial length thereof, and so reduces the contact between the bristle tips and the shaft; in turn this reduces the sealing efficiency.